sulfite

A space-filling model of the sulfite anion.

Sulfites (also sulphites) are compounds that contain the sulfite ion SO2−3 (additive IUPAC name: trioxidosulfate(2−)). The sulfite ion is the conjugate base of sulfurous acid. Although the acid itself is elusive, its salts are widely used.

Structure

The structure of the sulfite anion

The structure of the sulfite anion can be described with three equivalent resonance structures. In each resonance structure, the sulfur atom is double-bonded to one oxygen atom with a formal charge of zero (neutral), and sulfur is singly bonded to the other two oxygen atoms, which each carry a formal charge of −1, together accounting for the −2 charge on the anion. There is also a non-bonded lone pair on the sulfur, so the structure predicted by VSEPR theory is trigonal pyramidal, as in ammonia (NH₃). In the hybrid resonance structure, the S-O bonds are equivalently of bond order one and one-third.

Commercial uses

Wine

Sulfites occur naturally in all wines to some extent.^[1] Sulfites are commonly introduced to arrest fermentation at a desired time, and may also be added to wine as preservatives to prevent spoilage and oxidation at several stages of the winemaking.

Organic wines are not necessarily sulfite-free.^[2] In general, white and sweet (dessert) wines contain more sulfites than dry red wines.^[3]

In the United States, wines bottled after mid-1987 must have a label stating that they contain sulfites if they contain more than 10 parts per million.^[2]

In the European Union an equivalent regulation came into force in November 2005.^[4]

Other food

Sulfites are often used as preservatives in dried fruits, and dried potato products.

Most beers no longer contain sulfites. Although shrimp are sometimes treated with sulfites on fishing vessels, the chemical may not appear on the label. In 1986, the Food and Drug Administration in the United States banned the addition of sulfites to all fresh fruit and vegetables that are eaten raw.

Health risks

Sulfites are used in increasing amounts as a food preservative or enhancer. They may come in various forms, such as:^[5]

• Sulfur dioxide
• Potassium bisulfite or potassium metabisulfite
• Sodium bisulfite, sodium metabisulfite or sodium sulfite

Sulfites are counted among the top nine food allergens, but a reaction to sulfite is not a true allergy.^[6] It may cause breathing difficulty within minutes after eating a food containing it,^[7]. Asthmatics^[8][9] and possibly people with salicylate sensitivity (or aspirin sensitivity)^[10][11] are at an elevated risk for reaction to sulfites. The reaction can, but not often^{[citation needed]}, be fatal, and the individual must immediately go to the emergency room.^[7] Other symptoms include sneezing, swelling of the throat, and hives.^[11]

Labeling regulations don't require that products indicate the presence of sulfites in foods other than wine^{[citation needed]}; however, many companies voluntarily label sulfite-containing foods. Regulations do exist that require that ingredients lists show sulfites if they were added to a product, but this requirement applies only if they were intentionally added in formulation and not if they are contained in an ingredient. If a product includes an ingredient that contains sulfites, such as dried fruit, then the ingredients label will list only "dried fruit" and is not required to indicate whether the dried fruit itself contains sulfites. Furthermore, the products most likely to contain sulfites (fruits and alcoholic beverages) do not require ingredients labels, so the presence of sulfites is usually undisclosed.

Sulfites are widely used to extend the shelf life of products. Because it is often difficult to know whether a food contains sulfites, many people do not realize that they may have a sensitivity to sulfite. Sulfites are also known to destroy vitamin B1 (thiamin),^[12] a vitamin essential for metabolism of carbohydrates and alcohol.

Metabolic diseases

High blood and urine levels of sulphite in babies can be caused by molybdenum cofactor deficiency disease which leads to neurological damage and (until 2009) early death.

See also

• SO2−5 persulfate ion
• SO2−4 sulfate ion
• SO2−2 hyposulfite ion
• S₂O2−3 thiosulfate ion
• Category:Sulfites for a list of sulfites.
• Grant v The Australian Knitting Mills

References